Palladium fixing and low fresh oxygen storage capacity using tannic acid as a complexing and reducing agent

ABSTRACT

A method of manufacturing a catalyst article, the method comprising: providing a complex of a polyphenol and a PGM, the polyphenol comprising an ester functional group, the PGM comprising palladium; providing a support material; applying the complex to the support material to form a loaded support material; disposing the loaded support material on a substrate; and heating the loaded support material to form nanoparticles of the PGM on the support material.

FIELD OF THE INVENTION

The invention relates to a method of manufacturing a catalyst article, acatalyst article obtainable by the method, an emission treatment systemand a method of treating an exhaust gas.

BACKGROUND OF THE INVENTION

A three-way catalyst (TWC) allows simultaneous conversions (˜98%) of CO,HCs and NO_(x) from gasoline engine exhaust to innocuous compounds atstoichiometric air-to-fuel ratio. Specifically, the oxidation of CO andHCs to CO₂ and steam (H₂O) is mainly catalyzed by Pd, while thereduction of NO_(x) to N₂ is mainly catalyzed by Rh. Modern TWCs usesupported platinum group metal (hereinafter “PGM”) catalysts (Pd, Rh,Pt, etc.) deposited on a single, double or multilayer support, with thesupport material consisting of metal oxides with high specific surfacearea, primarily stabilized gamma alumina and ceria-containing oxygenstorage materials. The supported catalyst is washcoated on a ceramicmonolithic substrate.

Conventional preparation of a TWC washcoat slurry generally involves theuse of a solution of an inorganic PGM precursor, e.g. nitrate, acetate,hydroxide or chloride salt, to allow the PGM element to be depositedonto the oxide support via incipient wetness or wet impregnation.Promoter salts are also often added to the washcoat formulations forenhanced TWC performance. Once the monolithic substrate is washcoatedwith the as-prepared slurry, drying and calcination steps are followedto decompose the inorganic salts and to allow PGM and promoter elementsto be fixed onto the support materials. It is known that the performanceof supported metal catalysts depends on the structure and composition ofthe metal nanoparticles, and the nature of the support. ConventionalTWCs prepared using the above method often provide only limited controlover the structure of the catalytically active species (i.e. average PGMparticle size and composition, location of the active components, andmetal-support interactions). This is mainly due to metal migration andgrain growth during high temperature calcination process.

With increasingly stringent environmental regulations, TWCs with higheremissions abatement efficiency are needed. On the other hand, withincreasing PGM cost, there is an urgent need of reducing PGM loadingwithout compromising TWC performance. A better control of the PGMparticle size and metal-support interaction is essential in optimizingthe TWC performance. Furthermore, a uniformed PGM particle sizedistribution may contribute to a reduction in the extent of metalsintering due to Ostwald Ripening, as often occurs during a fuel cutoffprocess, an engine strategy used for enhanced fuel economy. Also, forPd-based TWCs with a Ba component as an additive, it is important tocontrol the locations of both palladium and barium for optimizing thesynergistic interactions with the active Pd, additive species, andsupport components. Moreover, in catalyst articles prepared byconventional methods, any additional oxygen storage capacity (OSC) thatmay be provided by Pd is very unstable, i.e. very high when fresh butdrops substantially after aging. This is thought to be due, in part, tosintering of the Pd during ageing and/or migration of the Pd, which inconventional methods is not generally very securely fixed to the supportmaterial, for example, thereby deactivating the OSC effect of thesupported Pd.

The catalyst light-off is the minimum temperature necessary to initiatethe catalytic reaction. In particular, the light-off temperature is thetemperature at which conversion reaches 50%. There is a need forcatalyst articles with reduced light-off temperatures.

US 2012/0077669 A1 describes a polymer-assisted synthesis of a supportedmetal catalyst for automotive applications. The polymers used in theexamples include poly(vinylpyrrolidone), poly(acrylic acid), andpoly(ethyleneimine). In the described synthesis procedures, the support(alumina powder) is first impregnated with a polymer-containing aqueoussolution. The impregnated support is then separated from the abovesolution by filtration and drying steps. The dried impregnated supportis further impregnated with a PGM precursor solution by incipientwetness impregnation. The as-described process involves multiple stepsfor the formation of as-claimed supported metal catalysts, whichincreases the cost and difficulty for commercial-scale production. US2012/0077669 A1 indicates that a lean burn engine is preferably used,such as a diesel engine or a lean burn gasoline engine, for theapplication of the technology.

SUMMARY OF THE INVENTION

One aspect of the present disclosure is directed to a method ofmanufacturing a catalyst article, the method comprising: providing acomplex of a polyphenol and a PGM, the polyphenol comprising an esterfunctional group, the PGM comprising palladium; providing a supportmaterial; applying the complex to the support material to form a loadedsupport material; disposing the loaded support material on a substrate;and heating the loaded support material to form nanoparticles of the PGMon the support material.

Another aspect of the present disclosure is directed to a catalystarticle obtainable by the method in the first aspect.

The invention also encompasses an exhaust system for internal combustionengines that comprises the catalyst article in the second aspect.

Another aspect of the present disclosure is directed to a method oftreating an exhaust gas, the method comprising: providing the catalystarticle of the second aspect; and contacting the catalyst article withan exhaust gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows an EPMA mapping image of Pd for Reference Example 1. FIG.1 b shows an EPMA mapping image of Pd for Example 1. FIG. 1 c shows anEPMA mapping image of Pd for Example 2. FIG. 1 d shows an EPMA mappingimage of Pd for Example 3.

FIG. 1 e shows an EPMA mapping image of Pd for Example 4.

FIG. 2 a shows an EPMA mapping image of Ba for Reference Example 1. FIG.2 b shows an EPMA mapping image of Ba for Example 1. FIG. 2 c shows anEPMA mapping image of Ba for Example 2. FIG. 2 d shows an EPMA mappingimage of Ba for Example 3.

FIG. 2 e shows an EPMA mapping image of Ba for Example 4.

FIG. 3 shows elemental correlation (Pd—Ba, Pd—Al, and Pd—Ce) extractedfrom EPMA images of Pd-TWC washcoats of Reference Example 1 and Examples1-4.

FIG. 4 shows oxygen storage capacity (OSC) of fresh and aged Pd-TWCwithout or with tannic acid modification.

FIG. 5 a shows NO_(x) conversion results of perturbated light-offperformance testing of Reference Example 1 and Examples 1-4. FIG. 5 bshows CO conversion results of perturbated light-off performance testingof Reference Example 1 and Examples 1-4. FIG. 5 c shows THC conversionresults of perturbated light-off performance testing of ReferenceExample 1 and Examples 1-4.

DETAILED DESCRIPTION OF THE INVENTION

The present invention seeks to tackle at least some of the problemsassociated with the prior art or at least to provide a commerciallyacceptable alternative solution thereto.

In a first aspect, the present invention provides a method ofmanufacturing a catalyst article, the method comprising:

-   -   providing a complex of a polyphenol and a PGM, the polyphenol        comprising an ester functional group, the PGM comprising        palladium;    -   providing a support material;    -   applying the complex to the support material to form a loaded        support material;    -   disposing the loaded support material on a substrate; and    -   heating the loaded support material to form nanoparticles of the        PGM on the support material.

Each aspect or embodiment as defined herein may be combined with anyother aspect(s) or embodiment(s) unless clearly indicated to thecontrary. In particular, any features indicated as being preferred oradvantageous may be combined with any other feature indicated as beingpreferred or advantageous.

Surprisingly, when used in an emission treatment system, the catalystarticle manufactured by the method of the present invention may exhibitfavourable catalytic activity, in particular favourable three-waycatalytic activity. For example, the catalyst article may exhibitfavourable light-off performance, in particular conversions of NO, COand total hydrocarbons, during three-way catalytic emissions abatementfor a stoichiometric gasoline engine. Such favourable catalytic activityand light-off performance may be superior to that exhibited byconventional catalyst articles with the same/similar PGM specie(s),loading(s), support(s), and configuration(s). The catalyst article maybe more durable in comparison to conventional catalyst articles. Inother words, such favourable catalytic activity may be exhibited evenafter aging.

Advantageously, such superior performance may facilitate the use oflower loadings of PGMs in comparison to conventional catalyst articleswithout compromising catalytic performance. This may be beneficial inview of the high cost of such metals, such as palladium. Furthermore,such superior performance may facilitate the partial/completesubstitution of high cost PGMs with lower cost PGMs or other transitionmetals without compromising catalytic performance.

Without being bound by theory, it is hypothesised that such superiorperformance may be provided by a favourable particle size distributionof the PGM nanoparticles on the support material. During PGM-polyphenolcomplexation, ions of PGM may react with ester functional groups, withthe same predictable amount of PGM ions “uptaken” by each polymer unitstructure, wherein the total amount of PGM “uptaken” is determined bythe polymer molecular structure/size and PGM-polyphenol coordinationratio. Each complex may then react/interact with surface functionalgroups (e.g. hydroxyl groups) or surface charges to allow “anchoring” ofPGM-polyphenol complexes onto the support material surface. The“anchored” PGM-polyphenol complexes may be separated apart due topolymeric steric effects and the available amount of support materialsurface functional groups/charges. The interaction between the complexand support material functional groups may increase PGM uptake by thesupport, compared to catalyst prepared by conventional methods. Withoutbeing bound by theory, it is also hypothesized that such even separationmay lead to a narrower particle size distribution of PGM nanoparticles(more uniformed particle sizes) upon heating (calcination), whichfurther leads to a reduction in excess agglomeration and/or sintering ofPGM particles during ageing and/or fuel-cut events. In other words,compared to conventional catalyst, a more sintering-resistant catalystarticle may be obtained by using the method of the present invention.

When used in an emission treatment system, surprisingly such a catalystarticle may provide lower oxygen storage capacity (OSC) and a smallerOSC difference between the fresh and aged catalyst article. A highloading of Pd (relative to Rh, for example) is generally required onsuch catalyst articles for TWC applications because of the relativelylow activity of Pd compared to Rh, for example. In addition to any OSCeffect of the support material, for example, this large amount of Pd mayalso contribute to the OSC effect. Without wishing to be bound by theorythis is thought to be due to reversible conversion between Pd and Pdoxide at engine operating temperatures. However, the OSC effect of Pd isvery unstable, i.e. very high when fresh but drops substantially afteraging. This is thought to be due, in part, to sintering of the Pd duringageing and/or migration of the Pd, which in conventional methods is notgenerally very securely fixed to the support material, for example,thereby deactivating the OSC effect of the supported Pd.

However, as described herein the method of the present invention mayprovide improved Pd fixing and Pd distribution. Advantageously, it hasbeen surprisingly found that this may result in a catalyst articlehaving a smaller OSC difference between the fresh and aged catalystarticle, which is thought to be due to a more stable OSC ability of thesupport Pd, for example. In other words, the OSC effect of the catalystarticle may be more stable throughout its lifetime, even after ageing.This may be very desirable in practice at least because enginecalibration (i.e. for the feedback loop between the engine and exhaustsystem in order to control the air-to-fuel ratio) is usually carried outusing the fresh catalyst article and is not re-calibrated after ageingor throughout the vehicle's lifetime. Therefore any difference betweenthe OSC ability of the fresh catalyst article and the aged catalystarticle may reduce the performance of the engine and exhaust system andmay therefore result in less efficient emissions reduction in theexhaust system. Thus, if the difference between the fresh and aged OSCability of the catalyst article is minimised, then the calibrationbetween the engine and the exhaust system may remain more accuratethroughout the lifetime of the vehicle, thereby helping to improve theefficiency of emissions reduction throughout the lifetime of thevehicle, even after ageing.

Moreover, due to the improved fixing of the Pd to the support materialthat may be provided by the method of the present invention,significantly less wicking of the Pd or washcoat layers through thesubstrate and/or mixing of the washcoat layers may be observed. In otherwords, catalyst articles manufactured by the method of the presentinvention may exhibit stronger/more reliable fixing of the PGMs to thesupport material compared to catalyst articles manufactured byconventional methods. As well as aesthetic improvements in such catalystarticles, the catalyst articles manufactured by the method of thepresent invention may therefore exhibit improved catalytic activity.This is at least because mixing of the catalytically-active PGMs betweenany distinct washcoat layers in the catalyst article may be reduced,which may thereby reduce the likelihood that any of the washcoat layersare deactivated. This, in turn, may help to maintain the catalyticactivity of the overall catalyst article as high as intended when thecatalyst article is fresh, as well as after ageing. Keeping the PGMs ofany distinct washcoat layers within their intended respective layers canbe important for maintaining their intended catalytic purpose, whetherfor oxidation or reduction, for example. For example, it is known thatdirect interactions between Pd and Rh can reduce the catalytic activityof the individual components, especially the catalytic functions of theRh component.

Furthermore, depending on the order of the steps and the order of theaddition of the support material, the method of the present inventionmay be used to fix the PGM, for example Pd, to any standard supportmaterial. In other words, if there are multiple different supportmaterials in the washcoat, then the method of the present invention maybe used to target the PGM, e.g. Pd, to the desired support material bycontrolling the order of the steps, such as whether the support materialis first combined with the PGM precursor or with the complex of apolyphenol and a PGM, for example.

In comparison to the method of US 2012/0077669 A1, the method of thepresent invention is a simpler and more efficient “one-pot” method,without the need for pH adjustment, for example. The method of thepresent invention does not require separate impregnation, filtration,and drying steps for depositing the polymer molecules onto the supportmaterial. By using the method of the present invention, the yield ofpolymer-support and PGM-polymer interactions may increase because eachpolymer molecule added is utilized for interactions. In contrast, in US2012/0077669 A1, only limited amount of polymers may stay on the supportafter the filtration and washing steps. Furthermore, the catalystarticle prepared by the method of the present invention may bespecifically used as a three-way catalyst for stoichiometric gasolineemissions abatement. In contrast, the catalyst article made by themethod of US 2012/0077669 A1 has a particular application in lean burndiesel or gasoline engines.

The term “catalyst article” used herein may encompass an article inwhich a catalyst is supported thereon or therein. The article may takethe form of, for example, a honeycomb monolith, or a filter, e.g. a wallflow filter or a flow-through filter. The catalyst article may be foruse in an emission treatment system, in particular an emission treatmentsystem for a gasoline engine, preferably a stoichiometric gasolineengine. The catalyst article may be for use in three-way catalysis.

Providing a complex of a polyphenol and a PGM typically involvesproviding the complex in solution, for example an aqueous or alcoholsolution. Providing a complex of a polyphenol and a PGM typicallyinvolves mixing inorganic PGM precursor(s) and polyphenol in pure orsolution form in an aqueous medium, for example mixing PGM nitrate andpolyphenol in water.

The term “polyphenol” as used herein encompasses polymers containingseveral hydroxyl groups on aromatic rings. Polyphenols are generallymoderately water-soluble compounds. Polyphenols may be natural orsynthetic, but in the present invention are preferably naturallyoccurring so as to render the method more environmentally friendly. Theterm “polyphenol” as used herein may encompass a polymer with a weightaverage molecular weight of 500 to 4,000 g/mol, greater than 12 phenolichydroxyl groups and with 5 to 7 aromatic rings per 1,000 g/mol. Thepolyphenol may be, for example, a phenolic acid (e.g. tannic acid,caffeic acid), a flavonoid (e.g. flavone, flavonol, flavanol, flavanone,isoflavone), a stilbene, or a lignin (polyphenols derived fromphenylalanine found in flax seed and other cereals).

The polyphenol comprises an ester functional group, typically acarboxylate ester functional group. The polyphenol preferably comprisestwo or more ester functional groups and/or carboxylate ester functionalgroups. The terms “ester functional group” and “carboxylate esterfunctional group” may encompass a functional group comprising a carboxylgroup bonded to an OR group, i.e.

The PGM comprises palladium. Palladium may be particularly suitable forcarrying out three-way catalysis. In addition, palladium is expensivemeaning that it would be advantageous to be able to provide similarlevels of catalytic activity for the same amount of metal.

Furthermore, the use of palladium in the method of the present inventionmay result in particularly favourable perturbated light-off performance.The PGM may be in the form of an alloy. In addition to palladium, thePGM may comprise other PGMs such as, for example, one or more ofrhodium, platinum, ruthenium, osmium and iridium.

The complex may have a PGM atom to ester group ratio of from 2:1 to1:10, preferably from 1:1 to 1:8, more preferably from 1:2 to 1:5. Thecomplex may have a palladium atom to ester group ratio of from 2:1 to1:10, preferably from 1:1 to 1:8, more preferably from 1:2 to 1:5.

The support material may be any material that is capable of supportingthe complex and nanoparticles thereon on therein. The support materialmay take any form, but is typically in the form of a powder, moretypically a high surface area powder. When the method of the presentinvention is used to prepare a catalysed filter, such as a wall flowfilter or flow-through filter, the support material will typically be inthe form of a powder having a D50 of, for example, from 0.1 to 30 μm,more typically from 0.5 to 25 μm as measured using TEM, even moretypically 1 to 20 μm. Such particle sizes may facilitate desirablerheological properties of a slurry used to coat the filter. The supportmaterial may function as a washcoat. The support material may be awashcoat or may be part of a washcoat.

The support material may also serve as an oxygen storage material, whichstores and releases oxygen respectively at fuel lean and fuel richconditions, for facilitating the three-way catalytic conversion.

Applying the complex to the support material typically involvescontacting the complex and support material in the presence of a solvent(typically water) so as to produce a slurry. The term “slurry” as usedherein may encompass a liquid comprising insoluble material, e.g.insoluble particles. The slurry may comprise (1) solvent; (2) solublecontent, e.g. unreacted polyphenol polymer, inorganic PGM and promoterprecursor(s), and PGM-polymer complex (outside of the support); and (3)insoluble content, e.g. support particles with and without interactionswith the polymer and metal precursors. The slurry is typically stirred,more typically for at least 10 minutes, more typically for at least 30minutes, even more typically for at least an hour. Increased contactingand/or stirring times may increase the amount of complex that is loadedonto the support material.

The term “loaded support material” as used herein may encompass asupport material that has the PGM-polyphenol complex loaded thereon(e.g. on the surface of a high-surface area metal oxide supportmaterial) and/or loaded therein (e.g. within the pores of a zeolitesupport material). The complex is typically fixed to the support, forexample by electrostatic forces, hydrogen bonds, coordinate bonds,covalent bonds, and/or ionic bonds. For example, in the case of anoxide, ester functional groups (e.g. carboxylate ester functionalgroups) in the polyphenol and surface hydroxyl groups on the support mayinteract through electrostatic forces or hydrogen-bond formation.

The term “substrate” as used herein may encompass, for example, aceramic or metallic honeycomb, or a filter block, e.g. a wall flowfilter or flow-through filter. The substrate may comprise a ceramicmonolithic substrate. The substrate may vary in its materialcomposition, size and configuration, cell shape and density, and wallthickness. Suitable substrates are known in the art.

Disposing the loaded support material on a substrate may be carried outusing techniques known in the art. Typically, the loaded supportmaterial is disposed on the substrate by pouring a slurry of the loadedsupport material into the inlet of the substrate using a specificmoulding tool in a predetermined amount. As discussed in more detailbelow, subsequent vacuum and drying steps may be employed during thedisposition step. When the support is a filter block, the loaded supportmaterial may be disposed on the filter walls, within the filter walls(if porous) or both.

Heating the loaded support material is typically carried out in an ovenor furnace, more typically a belt or static oven or furnace, typicallyin hot air at a specific flow from one direction. The heating maycomprise calcination. The heating may also comprise drying. The dryingand calcination steps may be continuous or sequential. For example, aseparate washcoat may be applied after the substrate is alreadywashcoated and dried with a previous washcoat. A washcoated substratecan also be dried and calcined using one continuous heating program ifcoating is completed. During the heating, the complex may at leastpartially, substantially or completely decompose. In other words, theligands of the complex, i.e. the polyphenol, are at least partially,substantially or completely removed or separated from the PGM, and areremoved from the final catalyst article. Particles of such separatedPGMs may then begin to form metal-metal and metal-oxide bonds. As aresult of the heating (calcination), the substrate is typicallysubstantially free of polyphenol, more typically completely free ofpolyphenol.

The term “nanoparticle” as used herein may encompass a particle having adiameter of from 0.01 nm to 100 nm as measured by TEM. The nanoparticlesmay be in any shape, e.g. a sphere, a plate, cubic, cylindrical,hexagonal or a rod, but are typically spherical. The largest dimensionof the nanoparticle (i.e. the diameter if the nanoparticle isspherical), will typically be from 0.5 to 10 nm, more typically from 1to 5 nm, as measured by TEM.

Following the heating step, the substrate is typically cooled, moretypically to room temperature. The cooling is typically carried out inair with or without cooling agent/media, typically without coolingagent.

The polyphenol preferably comprises tannic acid. The term “tannic acid”as used herein may encompass a mixture of polygalloyl glucoses orpolygalloyl quinic acid esters with the number of galloyl moieties permolecule ranging from 2 up to 12 depending on the plant source used toextract the tannic acid. Tannic acid may be a natural phenolic compoundand may be extracted from, inter alia, bark of oak, hemlock, chestnutand mangrove; the leaves of certain sumacs; and fruits of many plants.The term “tannic acid” as used herein may encompass a compoundconsisting of a central glucose ring and 10 galloyl groups, i.e.decagalloyl glucose, as shown by the following structural formula:

Such a compound has the IUPAC name1,2,3,4,6-penta-O-{3,4-dihydroxy-5-[(3,4,5-trihydroxybenzoyl)oxy]benzoyl}-D-glucopyranoseor2,3-dihydroxy-5-({[(2R,3R,4S,5R,6R)-3,4,5,6-tetrakis({3,4-dihydroxy-5-[(3,4,5-trihydroxyphenyl)carbonyloxy]phenyl}carbonyloxy)oxan-2-yl]methoxy}carbonyl)phenyl3,4,5-trihydroxybenzoate. Each of the five hydroxyl groups of theglucose molecule is esterified with a molecule of digallic acid.

Tannic acid may coordinate with metal ions through hydrogen bonds orcovalent bonds.

The PGM-tannic acid complex may be fixed onto supports, e.g. metal oxidesupports, in a washcoat, where the carboxylate ester functional groupsin the tannic acid ligands and surface hydroxyl groups on the supportmay interact through electrostatic forces or hydrogen-bond formation.

Since tannic acid is naturally occurring, the use of tannic acid to formthe PGM complex may be more environmentally friendly in comparison tothe use of other, non-naturally occurring ligands. Tannic acidfavourably has a decomposition point in the range 210 to 215° C. It isalso soluble in water (1 g of tannic acid dissolves in 0.35 ml of waterat standard temperature and pressure). The use of tannic acid may resultin particularly favourable perturbated light-off performance.

The polyphenol preferably has a weight average molecular weight M_(W) offrom 500 to 4,000, more preferably from 1,000 to 2,000 g/mol, even morepreferably from 1,600 to 1,800 g/mol measured by light scattering. Theweight average molecular weight Mw is determined by the formula:

${\overset{\_}{M}}_{w} = \frac{\sum_{i}{N_{i}M_{i}^{2}}}{\sum_{i}{N_{i}M_{i}}}$where N_(i) is the number of molecules of molecular mass M_(i). The useof such weight average molecular weights may result in particularlyfavourable perturbated light-off performance.

The polyphenol preferably has a number average molecular weight M_(n) offrom 500 to 4,000 g/mol measured by gel permeation chromatography (GPC).The number average molecular weight M_(w) is determined by the formula:

${\overset{\_}{M}}_{n} = \frac{\sum_{i}{N_{i}M_{i}}}{\sum_{i}N_{i}}$where again N_(i) is the number of molecules of molecular mass M_(i).The use of such number average molecular weights may result inparticularly favourable perturbated light-off performance.

The PGM comprises palladium. Preferably, the PGM consists essentiallyof, more preferably consists of palladium. Palladium is a particularlyexpensive PGM and forms particularly suitable complexes withpolyphenols, especially tannic acid. The use of such a metal in themethod of the present invention may result in particularly favourableperturbated light-off performance. The PGM may comprise predominantlypalladium, i.e. at least 50 wt. % palladium, typically at least 80 wt. %palladium, more typically at least 95 wt. % palladium, even moretypically at least 99 wt. % palladium based on the total weight of PGM.

After heating the loaded support material the substrate preferablycomprises from 50 g/ft³ to 200 g/ft³ of the PGM, more preferably from 80g/ft³ to 150 g/ft³ of the PGM. In other words, the concentration of PGMapplied to the substrate via the loaded support material may be suchthat after heating the loaded support material the substrate comprisesfrom 50 g/ft³ to 200 g/ft³ of the PGM, more preferably from 80 g/ft³ to150 g/ft³ of the PGM. Obtaining such a loading of the PGM on thesubstrate would be easily achieved by the skilled person by, forexample, using either a higher or lower concentration of the complex ofthe polyphenol and the PGM and/or a higher or lower PGM atom to estergroup ratio. In other words, it is well within the skilled person'scapabilities to provide a substrate via the method of the presentinvention with the desired level of PGM loading. For example, from 50g/ft³ to 200 g/ft³ of the PGM, more preferably from 80 g/ft³ to 150g/ft³ of the PGM may be applied to the support material in the step ofapplying the complex to the support material to form a loaded supportmaterial.

The support material preferably comprises an oxide, preferably one ormore of Al₂O₃ (aluminum oxide or alumina), SiO₂, TiO₂, CeO₂, ZrO₂,CeO₂—ZrO₂, V₂O₅, La₂O₃ and zeolites. The oxide is preferably a metaloxide. The support material more preferably comprises alumina, even morepreferably gamma-alumina. The support material preferably comprisesceria-zirconia. The support material preferably comprises alumina andceria-zirconia. The alumina and/or ceria-zirconia is preferably doped,more preferably with an oxide of one or more of lanthanum, neodymium,yttrium, niobium, praseodymium, hafnium, molybdenum, titanium, vanadium,zinc, cadmium, manganese, iron, copper, calcium, barium, strontium,caesium, magnesium, potassium, or sodium; even more preferably with anoxide of lanthanum, neodymium or yttrium. Such doped oxides areparticularly effective as support materials. Preferably, the dopant ispresent in the alumina and/or ceria-zirconia in an amount of from 0.001wt. % to 20 wt. %, and more preferably from 0.5 wt. % to 10 wt. %.

The support material is preferably in the form of a powder having a D90of from 0.1 to 25 μm, more preferably from 0.5 to 5 μm.

The loaded support material is preferably disposed onto the substrate inthe form of a slurry. A slurry is particularly effective at disposing amaterial onto a substrate, in particular for maximized gas diffusion andminimized pressure drop during catalytic conversion.

Providing the complex of a polyphenol and a PGM preferably comprisessynthesising the complex in situ in the slurry.

The slurry is preferably prepared by a method comprising:

-   -   contacting a PGM salt and a polyphenol in water to form the        complex of a polyphenol and a PGM in an aqueous solution, the        PGM salt comprising palladium; and    -   applying the complex to the support material to form a loaded        support material by contacting the support material with the        aqueous solution;    -   optionally adding one or more of an oxygen storage material,        preferably ceria-zirconia; a promoter salt; a binder; an acid or        a base; a thickening agent; and a reducing agent to the aqueous        solution. The optional step of adding one or more of an oxygen        storage material, preferably ceria-zirconia; a promoter salt; a        binder; an acid or a base; a thickening agent; and a reducing        agent to the aqueous solution may be undertaken during any of        the steps of said preferred method of preparing the slurry.

In an alternative preferred embodiment, the slurry may be prepared by amethod comprising:

-   -   contacting a support material and an aqueous solution of a PGM        salt to form a slurry comprising a PGM-salt-loaded support        material, the PGM salt comprising palladium;    -   applying the complex of a polyphenol and a PGM to the support        material to form a loaded support material by contacting the        slurry comprising the PGM-salt-loaded support material and a        polyphenol in water;    -   optionally adding one or more of an oxygen storage material,        preferably ceria-zirconia; a promoter salt; a binder; an acid or        a base; a thickening agent; and a reducing agent to the aqueous        solution or the slurry comprising the PGM-salt-loaded support        material. The optional step of adding one or more of an oxygen        storage material, preferably ceria-zirconia; a promoter salt; a        binder; an acid or a base; a thickening agent; and a reducing        agent to the aqueous solution may be undertaken during any of        the steps of said preferred method of preparing the slurry, but        is preferably performed during the step in which the support        material is supplied in each case.

Preferably, the steps of contacting a PGM salt and a polyphenol in waterand contacting the slurry comprising the PGM-salt-loaded supportmaterial and a polyphenol in water in each alternative method ofpreparing the slurry comprise allowing sufficient reaction time forcomplexation to occur between the polyphenol and the PGM cations, forexample such steps are typically carried out for at least 10 minutes,more typically for at least 30 minutes, even more typically for at leastan hour, preferably with stirring. Without wishing to be bound bytheory, it is thought that the step of applying the complex of apolyphenol and a PGM to the support material to form a loaded supportmaterial by contacting the slurry comprising the PGM-salt-loaded supportmaterial and a polyphenol in water in the second alternative method ofpreparing the slurry may be followed by subsequent reduction andprecipitation of the PGM metal species on the support material.

In other words, the method of preparing the slurry may comprise firstproviding the complex of a polyphenol and a PGM in an aqueous solutionfollowed by addition of the support and other optional components, oralternatively comprise first adding the support material and otheroptional components to an aqueous solution of a PGM precursor (i.e. PGMsalt) followed by addition of the polyphenol.

Such a “one-pot” preparation methods may be simplified and lower cost incomparison to conventional methods. It may also maximize utilization ofthe polymers.

In other words, the steps of providing a complex of a polyphenol and aPGM; providing a support material; applying the complex to the supportmaterial to form a loaded support material; and disposing the loadedsupport material on a substrate may comprise:

-   -   contacting a PGM salt and a polyphenol in water to form the        complex of a polyphenol and a PGM in an aqueous solution, the        PGM salt comprising palladium;    -   adding the support material to the aqueous solution to form a        slurry of loaded support material;    -   optionally adding one or more of an oxygen storage material,        preferably ceria-zirconia; a promoter salt; a binder; an acid or        a base; a thickening agent; and a reducing agent to the slurry;        and    -   disposing the slurry on the substrate.

Alternatively, the steps of providing a complex of a polyphenol and aPGM; providing a support material; applying the complex to the supportmaterial to form a loaded support material; and disposing the loadedsupport material on a substrate may comprise:

-   -   contacting a support material and an aqueous solution of a PGM        salt to form a slurry comprising a PGM-salt-loaded support        material, the PGM salt comprising palladium;    -   adding the polyphenol to the slurry comprising the        PGM-salt-loaded support material to form a slurry of loaded        support material;    -   optionally adding one or more of an oxygen storage material,        preferably ceria-zirconia; a promoter salt; a binder; an acid or        a base; a thickening agent; and a reducing agent to the aqueous        solution or the slurry comprising the PGM-salt-loaded support        material; and disposing the slurry on the substrate.

The loading may comprise washcoating.

The slurry preferably has a solids content of from 10 to 40%, preferablyfrom 15 to 35%. Such a solids content may enable slurry rheologiessuitable for disposing the loaded support material onto the substrate.For example, if the substrate is a honeycomb monolith, such solidcontents may enable the deposition of a thin layer of washcoat onto theinner walls of the substrate. If the substrate is a wall flow filter,such solids contents may enable the slurry to enter the channels of thewall flow filter and may enable the slurry to enter the walls of thewall flow filter.

Preferably, the slurry further comprises one or more of:

-   -   an oxygen storage material, preferably ceria-zirconia;    -   a promoter salt;    -   a binder;    -   an acid or a base;    -   a thickening agent; and    -   a reducing agent.

Promotors may include, for example, a non-PGM transition metal element,a rare earth element, an alkali or alkali earth group element, and/or acombination of two or more of the above elements within the same ordifferent groups in periodic table. The promotor salt may be a salt ofsuch elements. A particularly preferred promotor is barium, withparticularly preferred salts thereof being barium acetate, bariumcitrate and barium sulfate, or a combination thereof, more preferablybarium citrate.

Binders may include, for example, an oxide material with small particlesize to bind the individual insoluble particles together in washcoatslurry. The use of binders in washcoats is well known in the art.

Thickening agents may include, for example, a natural polymer withfunctional hydroxyl groups that interacts with insoluble particles inwashcoat slurry. It serves the purpose of thickening washcoat slurry forthe improvement of coating profile during washcoat coating ontosubstrate. It is usually burned off during washcoat calcination.Examples of specific thickening agents/rheology modifiers for washcoatsinclude glactomanna gum, guar gum, xanthan gum, curdlan schizophyllan,scleroglucan, diutan gum, Whelan gum, hydroxymethyl cellulose,carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose,methyl hydroxyethyl cellulose, methyl hydroxypropyl cellulose and ethylhydroxycellulose.

The term “reducing agent” as described herein may encompass a compoundthat can reduce the PGM cations to particles in its reduced state oreven metallic state in situ during washcoat preparation.

An organic acid can be added that acts as a reductant for PGM and/orcreates a reducing environment during the washcoat preparation atambient temperature or elevated temperature (<100° C.) within certainperiod of time. Examples of a suitable organic acid may include citricacid, succinic acid, oxalic acid, ascorbic acid, acetic acid, formicacid, tannic acid, and combinations thereof.

In a preferred embodiment, the support material comprises alumina andthe slurry further comprises ceria-zirconia. In another preferredembodiment, the support material comprises ceria-zirconia and the slurryfurther comprises alumina. In another preferred embodiment, the supportmaterial comprises alumina and ceria-zirconia.

The method preferably further comprises disposing a further slurry onthe substrate, the further slurry comprising one or more of a furthersupport material; an oxygen storage material; a promoter salt; a binder;an acid or a base; a thickening agent; and a reducing agent, whereindisposing the further slurry on the substrate takes place beforedisposing the support material on the substrate and/or after heating theloaded support material to form nanoparticles of the PGM on the supportmaterial. This may result in a catalyst article having multiple layersof different washcoats, for example a bottom washcoat containing, interalia, palladium nanoparticles supported on alumina, and a top washcoatcontaining, inter alia, palladium nanoparticles supported on alumina.Further examples of such multiple layers are discussed in more detailbelow.

Disposing the loaded support material on a substrate preferablycomprises contacting the slurry with the substrate (e.g. pouring theslurry into an inlet of the substrate) and optionally:

-   -   applying a vacuum to the substrate, and/or drying the slurry on        the substrate.

This may result in a favourable distribution of the loaded supportmaterial on the substrate.

The drying preferably occurs:

-   -   at a temperature of from 60° C. to 200° C., more preferably from        70° C. to 130° C.; and/or    -   for from 10 to 360 minutes, preferably from 15 to 60 minutes.

The substrate may be a “blank”, i.e. un-washcoated, substrate.Alternatively, the substrate may have one or washcoats already loadedthereon. In such a situation, the final catalyst article may comprisemultiple layers of different washcoats.

The substrate preferably comprises cordierite. Cordierite substrates areparticularly suitable for use in catalyst articles.

The substrate is preferably in the form of a honeycomb monolith, a wallflow filter or a flow through filter.

The heating is preferably carried out:

-   -   at a temperature of from 400° C. to 700° C., preferably from        400° C. to 600° C., more preferably from 450° C. to 600° C.;        and/or    -   for from 10 to 360 minutes, preferably from 35 to 120 minutes.

Lower temperatures and/or shorter heating times may result ininsufficient decomposition of the complex and/or may result in highlevels of polyphenol remaining in the substrate. Higher temperaturesand/or longer heating times may lead to the particles of PGM having anunfavourably large particle size, presumably due to sintering. Highertemperatures and longer heating times may also lead to damage to thecatalyst article.

The heating preferably comprises calcining. The term “calcining” as usedherein may encompass a thermal treatment process in the absence of, orlimited supply of, air or oxygen to bring about a thermal decomposition.

The nanoparticles preferably have a D50 of from 0.1 nm to 30 nm, morepreferably from 0.5 to 25 nm, even more preferably from 1 to 20 nm. TheD50 may be measured by TEM.

Such particle sizes may result in a favourable level of catalyticactivity.

In a further aspect, the present invention provides a catalyst articleobtainable by the method described herein, the catalyst article for usein an emission treatment system.

In comparison to conventional catalyst article, the catalyst articleobtainable by the method described herein may contain PGM particleshaving favourably larger particle sizes and a favourable particle sizedistribution (e.g. a D50 of from 1 to 20 nm) in their fresh state. Inaddition, in comparison to conventional catalyst articles, the catalystarticle obtainable by the method described herein may exhibit a moreuniformed dispersion of PGM particles throughout the substrate.

When used in an emission treatment system, the catalyst article mayexhibit favourable light-off performance, in particular for NO, CO andtotal hydrocarbons during three-way catalytic conversions forstoichiometric gasoline emissions abatement. The catalyst article mayalso exhibit the other favourable properties described herein, such as alower fresh OSC capacity, and less fresh-to-aged OSC difference (inanother word more stable OSC performance towards aging).

The catalyst is preferably for three-way catalysis.

The catalyst article may have a washcoat loading of from 1 g/in³ to 3g/in³. Such a catalyst article may exhibit similar or higher catalyticactivity in comparison to conventional catalyst articles but may belower cost in view of the lower levels of PGM employed.

The substrate preferably comprises a wall flow filter substrate or aflow-through substrate.

In a preferred embodiment, the catalyst article comprises a bottom layerof support material having rhodium thereon and a top layer of supportmaterial having palladium thereon. In such a catalyst article, forexample, the bottom layer may be provided by a method similar to thatdescribed herein or by any conventional method. In another preferredembodiment, the catalyst article comprises a bottom layer of supportmaterial having palladium thereon and a top layer of support materialhaving rhodium thereon. In such a catalyst article, for example, the toplayer may be provided by a method similar to that described herein or byany conventional method. The term “bottom layer” as used herein mayencompass a layer (e.g. washcoat layer) that is closest to or in contactwith the substrate (i.e. substrate walls). The term “top layer” as usedherein may encompass a layer (e.g. a washcoat layer) that is more remotefrom the substrate (i.e. substrate walls) than the bottom layer, and maybe situated on top of the bottom layer. In such layered catalystarticles, the top and/or bottom layer of support material may have afurther PGM thereon, for example platinum. In such layered catalystarticles, the top and/or bottom layer may comprise multiple PGMs, i.e.may be bimetallic (e.g. contain Pd—Rh or Pd—Pt) or trimetallic (e.g.Pd—Rh—Pt). The catalyst article may comprise two or more catalyst zones,for example an upstream zone and a downstream zone. The zones may differfrom each other by having different PGMs (e.g. Rh upstream and Pddownstream or vice versa) or differing by the amount of different typePGMs e.g. monometallic, bimetallic or trimetallic.

In such preferred embodiments, the support material preferably comprisesalumina and ceria-zirconia.

The catalyst article, in particular in such preferred embodiments,preferably comprises from 2 g/ft³ to 15 g/ft³ rhodium, more preferablyfrom 5 g/ft³ to 10 g/ft³ rhodium. Advantageously, such rhodium levelsmay be lower than those of conventional catalyst articles but withoutcompromising catalytic activity.

The catalyst article, in particular in such preferred embodiments,preferably comprises from 50 g/ft³ to 200 g/ft³ palladium, morepreferably from 80 g/ft³ to 150 g/ft³ palladium.

Advantageously, such palladium levels may be lower than those ofconventional catalyst articles but without compromising catalyticactivity.

In a preferred embodiment, the loaded support material is disposed onthe substrate in the form of a slurry, the PGM comprises palladium, thesupport material comprises alumina and the slurry further comprisesceria-zirconia. In another preferred embodiment, the loaded supportmaterial is disposed on the substrate in the form of a slurry, the PGMcomprises palladium, the support material comprises ceria-zirconia andthe slurry further comprises alumina. In another preferred embodiment,the loaded support material is disposed on the substrate in the form ofa slurry, the PGM comprises palladium and the support material comprisesalumina and ceria-zirconia.

In a further aspect, the present invention provides an emissiontreatment system comprising the catalyst article described herein.

The emission treatment system is preferably for a gasoline engine.

The gasoline engine preferably operates under stoichiometric conditions.

In a further aspect, the present invention provides a method of treatingan exhaust gas, the method comprising:

-   -   providing the catalyst article described herein; and    -   contacting the catalyst article with an exhaust gas.

The exhaust gas is preferably an exhaust gas from a gasoline engine. Thecatalyst article is particularly suitable for treating such exhaust gas.The gasoline engine preferably operates under stoichiometric conditions.

The invention will now be described in relation to the followingnon-limiting examples.

Manufacture of Catalyst Articles

A number of catalyst articles were prepared according to the followingexamples:

Reference Example 1: Pd/Ceria-Zirconia Mixed Oxide and Gamma Alumina(with Pd Nitrate) Washcoated TWC Catalyst

-   -   1. Prepare a solution with required amount of Pd nitrate (Pd        loading 131 g/ft³).    -   2. Add ceria-zirconia mixed oxide (1 g/in³) and gamma alumina (1        g/in), mix for 1 hr.    -   3. Add requirement amount of Ba acetate (Ba loading 400 g/ft³)        to the above slurry, mix for at least 30 min.    -   4. Adjust slurry solid content to 30%, add natrosol and mix        overnight.    -   5. Coat single dose targeting 1.2 inch, dry with pilot plant air        cure.    -   6. Fire the brick at 500° C. for 30 min in a static oven or a        rotating oven.

Example 1: Pd/Ceria-Zirconia Mixed Oxide and Gamma Alumina (with PdModified by Tannic Acid—High Tannic Acid Loading) Washcoated TWCCatalyst; Barium Acetate as the Barium Source

-   -   1. Prepare a solution with required amount of Pd nitrate (Pd        loading 131 g/ft³).    -   2. Add ceria-zirconia mixed oxide (1 g/in³) and gamma alumina (1        g/in³), mix for 1 hr.    -   3. Add required amount of tannic acid (232 g/ft³), targeting        molar ratio of Tannic acid:Pd=1:9. Mix for at least 2 hrs.    -   4. Add requirement amount of Ba acetate (Ba loading 400 g/ft³)        to the above slurry, mix for at least 30 min.    -   5. Adjust slurry solid content to 30%, add natrosol and mix        overnight.    -   6. Coat single dose targeting 1.2 inch, dry with pilot plant air        cure.    -   7. Fire the brick at 500° C. for 30 min in a static oven or a        rotating oven.

Example 2: Pd/Ceria-Zirconia Mixed Oxide and Gamma Alumina (with PdModified by Tannic Acid—Low Tannic Acid Loading) Washcoated TWCCatalyst; Barium Acetate as the Barium Source

-   -   1. Prepare a solution with required amount of Pd nitrate (Pd        loading 131 g/ft³).    -   2. Add ceria-zirconia mixed oxide (1 g/in³) and gamma alumina (1        g/in³), mix for 1 hr.    -   3. Add required amount of tannic acid (174 g/ft³), targeting        molar ratio of Tannic acid:Pd=1:12. Mix for at least 2 hrs.    -   4. Add requirement amount of Ba acetate (Ba loading 400 g/ft³)        to the above slurry, mix for at least 30 min.    -   5. Adjust slurry solid content to 30%, add natrosol and mix        overnight.    -   6. Coat single dose targeting 1.2 inch, dry with pilot plant air        cure.    -   7. Fire the brick at 500° C. for 30 min in a static oven or a        rotating oven.

Example 3: Pd/Ceria-Zirconia Mixed Oxide and Gamma Alumina (with PdModified by Tannic Acid—Low Tannic Acid Loading) Washcoated TWCCatalyst; Barium Citrate as the Barium Source

-   -   1. Prepare a solution with required amount of Pd nitrate (Pd        loading 131 g/ft³).    -   2. Add ceria-zirconia mixed oxide (1 g/in³) and gamma alumina (1        g/in³), mix for 1 hr.    -   3. Add required amount of tannic acid (174 g/ft³), targeting        molar ratio of Tannic acid:Pd=1:12. Mix for at least 2 hrs.    -   4. Add requirement amount of Ba acetate (Ba loading 400 g/ft³)        to the above slurry, mix for at least 30 min.    -   5. Add required amount of citric acid (CA loading 560 g/ft³) to        the above mixture, mix for at least 2 hrs.    -   6. Adjust slurry solid content to 30%, add natrosol and mix        overnight.    -   7. Coat single dose targeting 1.2 inch, dry with pilot plant air        cure.    -   8. Fire the brick at 500° C. for 30 min in a static oven or a        rotating oven.

Example 4: Pd/Ceria-Zirconia Mixed Oxide and Gamma Alumina (with PdModified by Tannic Acid—Low Tannic Acid Loading) Washcoated TWCCatalyst; Barium Sulfate as the Barium Source

-   -   1. Prepare a solution with required amount of Pd nitrate (Pd        loading 131 g/ft³).    -   2. Add ceria-zirconia mixed oxide (1 g/in³) and gamma alumina (1        g/in³), mix for 1 hr.    -   3. Add required amount of tannic acid (174 g/ft³), targeting        molar ratio of Tannic acid:Pd=1:12. Mix for at least 2 hrs.    -   4. Add requirement amount of Ba sulfate (Ba loading 400 g/ft³)        to the above slurry, mix for at least 30 min.    -   5. Adjust slurry solid content to 30%, add natrosol and mix        overnight.    -   6. Coat single dose targeting 1.2 inch, dry with pilot plant air        cure.    -   7. Fire the brick at 500° C. for 30 min in a static oven or a        rotating oven.        EPMA (Electron Probe Micro-Analyzer) Pd and Ba Elemental Mapping        Images

FIG. 1 a-1 e show EPMA mapping images of Pd of reference catalyst(Reference Example 1—FIG. 1 a ) vs. modified catalysts using tannic acid(TA) and with the addition of different Ba salts (FIG. 1 b-1 e ).Compared to (a) reference catalyst (Reference Example 1—FIG. 1 a ), moreuniformed Pd dispersions were shown when TA at (b) high loading (Example1—FIGS. 1 b ) and (c) low loading (Example 2—FIG. 1 c ) were used for Pdmodification. In (a) Reference Example 1, Pd accumulates to the surfacelayer of the washcoat.

By contrast, Pd-TWC modified by TA showed improved Pd distributionthroughout the entire depth of the washcoat. Pd distribution alsoslightly improved when (d) citric acid (Example 3—FIG. 1 d) was added tocomplex with Ba (as Barium acetate) in situ, and (e) when BaSO₄ (Example4—FIG. 1 e ) was used instead of Barium acetate.

FIGS. 2 a-2 e show EPMA mapping images of Ba of reference catalyst(Reference Example 1—FIG. 2 a ) vs. modified catalysts using TA and withaddition of different Ba salts (FIGS. 2 a-2 e ). Compared to (a)Reference Example 1 (FIG. 2 a ), (b)-(c) catalysts modified by TA(Examples 1-2—Examples 2b-2c) showed little effect in Ba fixing. Bycontrast, when (d) citric acid (Example 3—FIG. 2 d ) was added tocomplex with Ba, more uniformed Ba distribution was observed. (e) Baaddition as BaSO₄ (Example 4—FIG. 2 e ) also showed uniformed Badistribution but with significantly increased Ba particle size, sinceBaSO₄ was insoluble in water throughout the washcoat preparationprocess.

Elemental Correlation Extracted from EPMA Images

FIG. 3 shows elemental correlation (Pd—Ba, Pd—Al, and Pd—Ce) extractedfrom EPMA images of Pd-TWC washcoats of Reference Example 1 and Examples1-4. Compared to (a) Reference Example 1, TA modification in washcoats(b-d) Examples 1-3 significantly increased Pd correlation with Al,indicating increased Pd fixing on alumina. Additionally, Ba-citric acidinteraction in washcoat (d) (Example 3) promoted Pd—Ba interaction andPd fixing on ceria.

Oxygen Storage Capacity

FIG. 4 shows oxygen storage capacity (OSC) of fresh and aged Pd-TWCwithout (Reference Example 1) or with (Pd-TA) tannic acid modification.Compared to Reference Example 1, lower fresh OSC and lower fresh-to-ageddifference was observed with TA-modified catalyst (Example 1).

Perturbated Light-Off Performance

FIGS. 5 a-5 c shows the improved light-off performance, especiallylow-temperature NO_(x) conversion (FIG. 5 a ), CO (FIG. 5 b ) and THC(FIG. 5 c ) at all temperatures, of TA-modified Pd-TWC catalysts afteraging (Aging conditions: 1000° C./Redox/40 hr. Reaction conditions: withrich pre-treatment, 150-700° C., A=0.96-1.04, GHSV=200,000 hr⁻¹).Compared to (a) Reference Example 1, (b)-(d) Examples 1-3 showed thatimproved TWC light-off performance can be achieved by (1) optimizing TAloading, and (2) optimizing interaction between Pd and additive, e.g.barium species. Optimization of sequence of addition is also importantfor performance improvement of TA-modified Pd-TWC

The foregoing detailed description has been provided by way ofexplanation and illustration, and is not intended to limit the scope ofthe appended claims. Many variations in the presently preferredembodiments illustrated herein will be apparent to one of ordinary skillin the art and remain within the scope of the appended claims and theirequivalents.

The invention claimed is:
 1. A method of manufacturing a catalystarticle, the method comprising: providing a complex of a polyphenol anda PGM, the polyphenol comprising an ester functional group, the PGMcomprising palladium; providing a support material; applying the complexto the support material to form a loaded support material; disposing theloaded support material on a substrate; and heating the loaded supportmaterial to form nanoparticles of the PGM on the support material. 2.The method of claim 1, wherein the polyphenol comprises tannic acid. 3.The method of claim 1, wherein the PGM consists of palladium.
 4. Themethod of claim 1, wherein the support material comprises an oxideselected from a group consisting of one or more of Al₂O₃, SiO₂, TiO₂,CeO₂, ZrO₂, CeO₂—ZrO₂, V₂O₅, La₂O₃ and zeolites.
 5. The method of claim1, wherein the support material comprises Al₂O₃ and CeO₂—ZrO₂.
 6. Themethod of claim 5, wherein the Al₂O₃ and/or CeO₂—ZrO₂ is doped.
 7. Themethod of claim 6, wherein the Al₂O₃ and/or CeO₂—ZrO₂ is doped with anoxide of one or more of lanthanum, neodymium, yttrium, niobium,praseodymium, hafnium, molybdenum, titanium, vanadium, zinc, cadmium,manganese, iron, copper, calcium, barium, strontium, caesium, magnesium,potassium and sodium, preferably one or more of lanthanum, neodymium andyttrium.
 8. The method of claim 1, wherein the loaded support materialis disposed on the substrate in the form of a slurry.
 9. The method ofclaim 8, wherein the slurry is prepared by a method comprising:contacting a PGM salt and a polyphenol in water to form the complex of apolyphenol and a PGM in an aqueous solution, the PGM salt comprisingpalladium; applying the complex to the support material to form a loadedsupport material by contacting the support material with the aqueoussolution; optionally, adding one or more of an oxygen storage material,preferably ceria-zirconia; a promoter salt; a binder; an acid or a base;a thickening agent; and a reducing agent to the aqueous solution. 10.The method of claim 8, wherein the slurry is prepared by a methodcomprising: contacting a support material and an aqueous solution of aPGM salt to form a slurry comprising a PGM-salt-loaded support material,the PGM salt comprising palladium; applying the complex of a polyphenoland a PGM to the support material to form a loaded support material bycontacting the slurry comprising the PGM-salt-loaded support materialand a polyphenol in water; optionally, adding one or more of an oxygenstorage material, preferably ceria-zirconia; a promoter salt; a binder;an acid or a base; a thickening agent; and a reducing agent to theaqueous solution or the slurry comprising the PGM-salt-loaded supportmaterial.
 11. The method of claim 8, wherein the support materialcomprises alumina and ceria-zirconia.
 12. The method of claim 8, furthercomprising disposing a further slurry on the substrate, the furtherslurry comprising one or more of a further support material; an oxygenstorage material; a promoter salt, preferably barium acetate, bariumsulfate, barium citrate or a combination thereof; a binder; an acid or abase; a thickening agent; and a reducing agent, wherein disposing thefurther slurry on the substrate takes place before disposing the supportmaterial on the substrate and/or after heating the loaded supportmaterial to form nanoparticles of the PGM on the support material. 13.The method of claim 8, wherein disposing the loaded support material ona substrate comprises contacting the slurry with the substrate andoptionally: applying a vacuum to the substrate, and/or drying the slurryon the substrate.
 14. The method of claim 1, wherein the substrate is inthe form of a honeycomb monolith, a wall-flow filter or a flow-throughfilter.
 15. The method of claim 1, wherein the heating is carried out:at a temperature of from 400° C. to 700° C., preferably from 400° C. to600° C., more preferably from 450° C. to 600° C.; and/or for from 10 to360 minutes, preferably from 35 to 120 minutes.